Cyclic Indentation Tests Research Articles

Inescapable foot shock induces a PTSD-like phenotype and negatively impacts adult murine bone.

Post-traumatic stress disorder (PTSD) is associated with osteopenia, osteoporosis and increased fracture risk in the clinical population. Yet, the development of preclinical models to study PTSD-induced bone loss remains limited. In this study, we present a previously unreported model of PTSD in adult female C57BL/6 mice, by employing inescapable foot shock and social isolation, that demonstrates high face and construct validity. A subset of mice exposed to this paradigm (i.e. PTSD mice) display long-term alterations in behavioral and inflammatory indices. Using three-dimensional morphometric calculations, cyclic reference point indentation (cRPI) testing and histological analyses, we find that PTSD mice exhibit loss of trabecular bone, altered bone material quality, and aberrant changes in bone tissue architecture and cellular activity. This adult murine model of PTSD exhibits clinically relevant changes in bone physiology and provides a valuable tool for investigating the cellular and molecular mechanisms underlying PTSD-induced bone loss.

Investigation the temperature-dependent surface mechanical properties of PolyJet printed samples by cyclic indentation testing in a DMA system

Tools and products used in everyday life are regularly exposed to dynamic, cyclical stress. These stresses are particularly significant in the case of polymers, as their viscoelastic behavior means that creep can be considerable even at room temperature. Therefore, polymer components are often tested for maximum deformation rather than for maximum load-bearing capacity. Basic material testing methods are tensile, compression, and flexure tests, which are excellent for determining the material properties of the bulk material. However, in some cases, it is more appropriate to investigate surface mechanical properties. The ideal method for this is the depth-sensing indentation test. This method has been widely applied to polymers in the last ten years, but cyclic tests are rare, and creep is not considered in this type of indentation test. In the case of cyclical tests, temperature dependence is not fully examined, either. This paper presents the results of cyclic indentation tests on photopolymer specimens produced by additive manufacturing with the use of a dynamic mechanical analyzer with a unique indentation clamp. We performed the tests at three temperatures to investigate the influence of temperature on cyclic indentation tests. The test results show the variation of deformation between cycles and the significant effect of temperature.

Nano-sized Cu precipitation and microstructural evolution in aged ultralow and medium carbon steels

The nanoscale Cu precipitation exhibits considerable potential for enhancing strength and cyclic hardening potential in steels. In the present work, Cu-alloyed ferritic steels with different C contents (X0.5: 0.005 wt.%, X21: 0.21 wt.%) were investigated to systematically manifest the influences of C contents on the microstructural evolution, the precipitation behavior of nano-sized Cu precipitates and the resulting effects on cyclic hardening behavior. The investigated materials were subjected to austenitization at 900 °C for 10 min and subsequently aged at 600 °C for different time durations (120, 720, 7200 s). The 3D atom probe tomography rendered the investigation of nanoscale Cu precipitates in terms of average size, local chemistry, number density and spatial distribution. In X21 steel, the Cu precipitates exhibited a large nucleation density upon the onset of aging and further grew into a larger precipitate size. Compared to X0.5 steel, the higher nucleation density and the inhomogeneous precipitate distribution in X21 steel were attributed to the heterogeneous nucleation and high C content in the matrix. Cyclic indentation tests revealed a strong dependency of hardness and cyclic hardening potential on precipitation size and number density for both steels. Furthermore, for both steels aging had a similar effect on the mechanical properties. However, the increased C content induced a higher hardness and lower cyclic hardening exponentCHT of X21 steel.

Accumulation of fluorescent advanced glycation end products and carboxymethyl-lysine in human cortical and trabecular bone

Chemical crosslinks known as advanced glycation end-products (AGEs) are associated with increased bone fracture risk and deteriorated bone mechanical properties. However, measurement of bone AGEs via ex vivo and in vitro methods has been limited to quantification of bulk fluorescent AGEs (fAGEs) and pentosidine only, which is a crosslinking fluorescent AGE. However, a non-crosslinking and non-fluorescent AGE such as carboxymethyl-lysine (CML) is found to be 40–100 times higher in quantity than pentosidine, but only one previous study has reported it in cortical bone, and one study reported it in trabecular bone. In our study, we wanted to investigate if accumulation of CML differs in cortical and trabecular compartments and if they are more strongly associated with bone mechanical properties than with fAGEs. We hypothesized that CML and fAGEs level would be higher in the trabecular compartment and show negative correlations to mechanical properties in cortical and trabecular bone. We obtained human cadaveric cortical and trabecular bone specimens, induced the formation of AGEs via the established in vitro ribosylation method, imaged specimens by microcomputed tomography to assess specimen geometry and microarchitecture, and mechanically tested cortical specimens by cyclic reference point indentation and fracture toughness tests and trabecular specimens by compression tests, followed by measurement of fAGEs and CML. fAGEs were 22 % higher in cortical bone (687 ± 44.8 ng Q/mg collagen) compared to trabecular bone (859 ± 317.1 ng Q/mg collagen), whereas CML levels were found to be 148 % higher in trabecular bone (6189.9 ± 866 ng/mg of protein) compared to cortical bone (924.6 ± 576.3 ng/mg of protein). Pooling the specimens from both the control and ribose groups, Spearman correlation analysis indicated that CML levels, but not fAGEs, are moderately associated with cortical porosity (r = +0.505, p ≤ 0.05) and mechanical properties such indentation depth (r = +0.460, p ≤ 0.05), total indentation depth (r = +0.440, p ≤ 0.05), and average energy dissipated (r = +0.465, p ≤ 0.05) in cortical bone. fAGEs showed a trend towards negative association with crack propagation toughness in cortical bone (r = −0.365, p = 0.055). No significant correlations were observed between CML and microarchitecture or mechanical properties in trabecular bone. CML levels were also associated with fAGEs in cortical bone (r = +0.596, p ≤ 0.05) but not in trabecular bone. Our preliminary findings indicate that CML, a non-crosslinking AGE, may affect bone material and mechanical properties differently than bulk fluorescent AGEs, given the higher accumulation of CML in each bone compartment. This study provides direction to future studies to quantify crosslinking and non-crosslinking AGEs separately as their effect on material and mechanical properties may be different and it would help identify better biomarkers for bone strength prediction.

Evaluation of the plastic deformation behavior of modified 100Cr6 steels with increased fractions of retained austenite using cyclic indentation tests

Abstract In highly loaded components, such as roller bearings, early failures may occur due to microstructural defects. Thus, a higher defect tolerance of the material can improve the fatigue lifetime. To produce steels with high defect tolerance, innovative alloying concepts, and a sound knowledge of the influence of heat treatment parameters on the resultant mechanical properties is indispensable. Consequently, two bearing steels based on 100Cr6 with different silicon contents were investigated to achieve relatively high retained austenite (RA) fractions, which are assumed to increase the defect tolerance at cyclic loading. To understand the relation between bainitic heat treatment and mechanical characteristics, the resultant RA fractions, Vickers hardness, microhardness, and cyclic hardening exponent CHT eII, which correlates to defect tolerance of metallic materials, were determined for differently heat-treated conditions. Based on the statistical design of experiments and measurements obtained with X-ray diffraction and cyclic indentation testing, regression calculations were conducted. Pertaining to the results of the regression analyses, the heat treatment parameterizations of these steels were specifically determined for certain desired property combinations within the boundaries investigated. Therefore, the temperatures used in the austenitizing and bainitizing procedures have shown the highest influence on the mechanical characteristics and the RA fraction of both steels.

A New Approach to Estimate the Fatigue Limit of Steels Based on Conventional and Cyclic Indentation Testing

For a reliable design of structural components, valid information about the fatigue strength of the material used is a prerequisite. As the determination of the fatigue properties, and especially the fatigue limit σw, requires a high experimental effort, efficient approaches to estimate the fatigue strength are of great interest. Available estimation approaches using monotonic properties, e.g., Vickers hardness (HV), and in some cases the cyclic yield strength, only allow a rough estimation of σw. The approaches solely based on monotonic properties lead to substantial deviations of the estimated σw in relation to the experimentally determined fatigue limit as they do not consider the cyclic deformation behavior. In this work, an estimation approach was developed, which is based on a correlation analysis of the fatigue limit σw, HV, and the cyclic hardening potential obtained in instrumented cyclic indentation tests (CIT). For this, eleven conditions from five different low-alloy steels were investigated. The CIT enable an efficient and quantitative determination of the cyclic hardening potential, i.e., the cyclic hardening exponentCHT eII, and thus, the consideration of the cyclic deformation behavior in an estimation approach. In this work, a strong correlation of σw with the product of HV and |eII| was observed. In relation to an existing estimation approach based solely on HV, considering the combination of HV and |eII| enables the estimation of σw with an enormously increased precision.

Correlation between microstructure and nanomechanical properties of 9Cr– 1Mo ferritic martensitic steel through instrumented indentation technique

Instrumented indentation technique (IIT) has been used to probe the phase-specific nanomechanical properties of heat treated 9Cr 1Mo ferritic martensitic (P9) steel. P9 steel was subjected to austenitization treatment, followed by water quenching, normalizing, furnace cooling, and subsequently tempering. Quantitative TEM analysis showed systematic changes in microstructure with variation in cooling rate in aforementioned heat treatments. Instrumented indentation tests were carried out at room temperature with a spherical indenter for evaluating hardness and elastic modulus. The yield strength of the steel from the load-displacement curves of cyclic indentation tests has been derived using neural networking method and yield parameter calculations. The properties derived from IIT using these methods were also validated with tensile test results. This work aims at correlating the local mechanical response measured by IIT with microstructural changes due to different heat treatments, which would be useful to extract mechanical properties of materials with heterogeneous microstructure.

Gradients of cyclic indentation mechanical properties in PR520 epoxy and its 3D carbon fiber composite induced by aging at 150 °C

A cyclic indentation test was carried out on PR520 epoxy resin and its 3D interlock carbon fiber reinforced composite in order to study the effects of thermo-oxidative aging on the material local properties. Thermal aging has been performed in air at 150 °C, which is close to the service temperature in warm zones of aircrafts, for different durations. Two parameters have been considered in this study: the reload indentation modulus, representative of the elastic response, and the ratio between irreversible and total indentation energy, representative of plastic (first cycle) and viscoelastic response. The analysis of cyclic indentation curves shows that elastic and plastic behavior are affected by thermal aging, while viscoelastic behavior is not. Moreover, a gradient of local properties has been measured through the thickness of aged samples. The affected zone extends of about 150 µm from the surface exposed to the oxidative environment, while the properties of the sample core are similar to those of the virgin material. Property gradient profiles measured on resin and composite aged in the same conditions are well superposed.

Analysis of the Subsurface Volume of Differently Finished AISI 52100 by Cyclic Indentation and X‐Ray Diffraction

Finishing processes result in changes of near‐surface morphology, which strongly influences the fatigue behavior of components. Especially, roller bearings show a high dependency of the lifetime on surface roughness and the residual stress state in the subsurface volume. To analyze the influence of different finishing processes on the near‐surface morphology, including the residual stress state, roller bearing rings made of AISI 52100 are finished in this work using hard turning, rough grinding, and fine grinding. In addition, fatigue specimens made of AISI 52100 and finished by cryogenic hard turning are investigated. For each condition, the residual stresses are determined at different distances from the surface, showing pronounced compressive stresses for all conditions. While the ground roller bearing rings show highest compressive residual stresses at the surface, the hard turned bearing ring and the cryogenic hard turned fatigue specimens reveal maximum compressive stresses in the subsurface volume. Moreover, cyclic indentation tests (CITs) are conducted in the different subsurface volumes, showing a higher cyclic plasticity in relation to the respective initial state, which is assumed to be caused by finishing‐induced compressive residual stresses. Thus, the presented results indicate a high potential of CITs to efficiently characterize the residual stress state.

Analyzing the influence of a deep cryogenic treatment on the mechanical properties of blanking tools by using the short-time method PhyBaLCHT

Deep cryogenic treatment (DCT) is a promising post-process for increasing the service lifetime of highly loaded tools. However, the process parameters as well as the position of DCT during thermal post-processing have to be adapted on the materials used, leading to high experimental effort, costs and time to optimize and validate this kind of thermal treatment. Therefore, efficient methods to analyze the mechanical properties of materials treated with DCT are of great industrial and scientific interest. In this context, the short-time method PhyBaLCHT, which is based on instrumented cyclic indentation tests, is a promising approach, as it enables a rapid determination of the cyclic deformation behavior and hardness of a material. Consequently, in the presented work different material conditions of the thermally treated tool steels 1.2379 (X155CrVMo12-1; AISI D2) and Vanadis 4E (140CrMoVMnSi19-35-37) were characterized by using cyclic indentation testing. The different conditions were realized by performing different sequences of thermal post-processing, containing quenching, tempering and DCT. The results obtained in cyclic indentation tests show for both materials slightly lower hardness but significantly higher cyclic plasticity of the material conditions treated with DCT antecedent to the tempering process compared to conventionally quenched and tempered material. In additionally performed blanking tests, this further results in an increased wear resistance of cryogenically treated conditions. Because of the higher cyclic plasticity, the reduction of local stress intensities at the cutting edges caused by plastic deformation is more pronounced for the cryogenically treated tools, which results in decreased wear rate as well as less pronounced breaking of the cutting edges. Consequently, the presented results demonstrate that the PhyBaLCHT method is a powerful means to characterize the mechanical properties of cryogenically treated materials and enables an estimation of the wear characteristics of highly loaded tools.

Analysis of Hydrogen-Induced Changes in the Cyclic Deformation Behavior of AISI 300–Series Austenitic Stainless Steels Using Cyclic Indentation Testing

The locally occurring mechanisms of hydrogen embrittlement significantly influence the fatigue behavior of a material, which was shown in previous research on two different AISI 300-series austenitic stainless steels with different austenite stabilities. In this preliminary work, an enhanced fatigue crack growth as well as changes in crack initiation sites and morphology caused by hydrogen were observed. To further analyze the results obtained in this previous research, in the present work the local cyclic deformation behavior of the material volume was analyzed by using cyclic indentation testing. Moreover, these results were correlated to the local dislocation structures obtained with transmission electron microscopy (TEM) in the vicinity of fatigue cracks. The cyclic indentation tests show a decreased cyclic hardening potential as well as an increased dislocation mobility for the conditions precharged with hydrogen, which correlates to the TEM analysis, revealing courser dislocation cells in the vicinity of the fatigue crack tip. Consequently, the presented results indicate that the hydrogen enhanced localized plasticity (HELP) mechanism leads to accelerated crack growth and change in crack morphology for the materials investigated. In summary, the cyclic indentation tests show a high potential for an analysis of the effects of hydrogen on the local cyclic deformation behavior.

Process-influenced fatigue behavior of AISI 316L manufactured by powder- and wire-based Laser Direct Energy Deposition

Because of the enormous potential of Laser Direct Energy Deposition (L-DED) regarding the production and maintenance of components with complex geometries, this type of Additive Manufacturing processes is of great industrial and scientific interest. As two principals of L-DED, i.e., wire-based (L-DED-W) and powder-based (L-DED-P) processes, are commonly used, it is indispensable to thoroughly analyze the influence of the raw material as well as process conditions on the resulting material properties. Therefore, in the present work specimens made of AISI 316L and manufactured via L-DED-P and L-DED-W were investigated. To characterize the cyclic properties of the produced material volume, instrumented cyclic indentation tests (CITs) as well as uniaxial fatigue tests were performed. The cyclic deformation behavior obtained in fatigue tests indicate a significantly higher fatigue strength of L-DED-W material, correlating with a higher δ-ferrite fraction and smaller grain size. This is caused by the different process conditions, whereby the increased δ-ferrite fraction of L-DED-W results from the difference in chemical composition. However, the S-Nf curves show a higher fatigue limit at 2 × 106 cycles for L-DED-P, which is caused by the significantly larger process-induced nonmetallic inclusions observed in L-DED-W specimens. In summary, the present work shows significant differences between the material produced with L-DED-P and L-DED-W, and demonstrates a strong influence of process-induced defects on the fatigue behavior of additively manufactured materials.

Investigating the Influence of Process Parameters on the Mechanical Properties of Extruded Aluminum Tubes by Cyclic Indentation Tests

Given the complex process condition, extruded aluminum (Al) alloy tubes show locally pronounced differences in microstructure and mechanical properties, which can be influenced by subsequent heat treatment. In the present study, cyclic indentation tests (CITs) were conducted on extruded Al alloy EN AW-6082 to locally determine hardness and cyclic hardening potential, which was complemented with light optical microscopy. To analyze the influence of extrusion process and subsequent heat treatment, the EN AW-6082 tubes investigated were manufactured with extrusion ratios Ψ of 13:1 and 22:1, both in as-extruded and T6 heat-treated conditions. The results obtained for the as-extruded state showed significant differences of the local mechanical properties and demonstrated that an increased Ψ leads to higher hardness, caused by more pronounced plastic deformation during the manufacturing process. Moreover, an increase of hardness and cyclic hardening potential was observed after a T6 heat treatment, which also reduced the difference in hardness between the different extrusion ratios. Additionally, the pronounced local differences in hardness and cyclic hardening potential correlated with the local microstructure. The results demonstrated that CITs enable the analysis of local mechanical properties of extruded EN AW-6082 profiles, resulting from different extrusions ratios as well as subsequent heat treatment.

Cyclic Indentation Test Rig Using Voice Coil Motor for DLC Films and Evalution of Indentaion Impression

Cyclic Indentation Test Rig Using Voice Coil Motor for DLC Films and Evalution of Indentaion Impression

Influence of Cu precipitates and C content on the defect tolerance of steels

Understanding the effects of microstructural defects, which can have a high impact on service life, is of great importance in the design of cyclically loaded components. For this, the size of the defect as well as the material’s ability to counteract microstructural defects have to be considered. In this study, the defect tolerance of differently heat-treated Cu alloyed steels with two different C contents (0.005 and 0.21 wt% C) has been investigated by testing specimens with artificial defects. From the results of fatigue experiments, defect tolerance is assessed based on different approaches: (i) a comparison of stress intensities, derived from Murakami’s √area approach, (ii) comparing fatigue lifetime for a given defect size and stress amplitude, (iii) the reduction of fatigue strength by a defect for a given defect size, and (iv) using the Kitagawa-Takahashi diagram, enabling the calculation of the critical defect size √area0 as well as fatigue crack propagation threshold ΔKth,calc. The results show that for the 0.005 wt% C steel an increase in aging time is accompanied by an increase in defect tolerance, which correlates with a higher cyclic hardening potential induced by the formation of Cu precipitates. However, the results show that strength and ductility also influence defect tolerance. By using the short-time procedure PhyBaLCHT, which is based on cyclic indentation tests, all these factors influencing defect tolerance can be determined efficiently. The 0.21 wt% C steel does not reveal an improved defect tolerance which is presumably caused by the dominating pearlite phase.

Analysis of the Thermomechanical Fatigue Behavior of Fully Ferritic High Chromium Steel Crofer®22 H with Cyclic Indentation Testing

The 22 wt.% Cr, fully ferritic stainless steel Crofer®22 H has higher thermomechanical fatigue (TMF)- lifetime compared to advanced ferritic-martensitic P91, which is assumed to be caused by different damage tolerance, leading to differences in crack propagation and failure mechanisms. To analyze this, instrumented cyclic indentation tests (CITs) were used because the material’s cyclic hardening potential—which strongly correlates with damage tolerance, can be determined by analyzing the deformation behavior in CITs. In the presented work, CITs were performed for both materials at specimens loaded for different numbers of TMF-cycles. These investigations show higher damage tolerance for Crofer®22 H and demonstrate changes in damage tolerance during TMF-loading for both materials, which correlates with the cyclic deformation behavior observed in TMF-tests. Furthermore, the results obtained at Crofer®22 H indicate an increase of damage tolerance in the second half of TMF-lifetime, which cannot be observed for P91. Moreover, CITs were performed at Crofer®22 H in the vicinity of a fatigue crack, enabling to locally analyze the damage tolerance. These CITs show differences between crack edges and the crack tip. Conclusively, the presented results demonstrate that CITs can be utilized to analyze TMF-induced changes in damage tolerance.

In-situ characterization of the local mechanical behaviour of polymer matrix in 3D carbon fiber composites by cyclic indentation test

In this work, a cyclic indentation test is employed for the in-situ characterisation of the local mechanical properties of a PR520 epoxy resin matrix in a 3D carbon fiber composite. Since the evolution of indentation response with time is studied, the cyclic loading allows to characterise both the elastic and the time dependent, viscoelastic, mechanical response of the material simultaneously. The 3D carbon fiber composite used in this study contains a number of large resin pockets between fiber bundles (mesoscopic scale) with a characteristic dimension ranging from several hundred of micrometers to several millimeters. The mechanical behaviour of the polymer matrix is investigated on the external surface and in volume of the composite and compared with the neat polymer. The instantaneous elastic modulus from reloading, the energy ratio and the residual depth are determined from the cyclic material response and compared through a Student t-test based statistical analysis. Results show that there is at least 95% of probability that the neat and in-situ polymer matrix material belong to different populations. However, this difference is rather small (between 1 and 2.5%) and almost constant with cycles. Moreover, a difference between composite core and surface was measured.

Influence of different precipitation states of Cu on the quasi-static and cyclic deformation behavior of Cu alloyed steels with different carbon contents

Cu precipitates in steels can lead to significant changes of mechanical behavior. However, most investigations focus on the influence of these precipitates on hardness, whereas the effects on quasi-static deformation behavior and cyclic properties remain unclear. Therefore, in the present work the deformation behavior of Cu alloyed steels with two different carbon contents, 0.005 and 0.2 wt%, was analyzed in tensile as well as fatigue tests. To characterize the influence of different precipitation states, various heat treatments were performed. The heat treatment parameters were chosen based on results obtained with the short-time procedure PhyBaLCHT, which is based on cyclic indentation tests and enables the determination of microhardness and cyclic hardening potential. The results reveal that a higher C content increases hardness, decreases cyclic hardening potential and leads to generally higher tensile strength. However, only at a shorter aging time does a higher C content increase fatigue strength, whereas at a longer aging time both steel variants show comparable fatigue lifetimes, despite the significantly higher hardness and tensile strength of the steel with higher carbon content. This can be explained with the higher cyclic hardening potential of the lower carbon steel, leading to an improved fatigue lifetime. Additionally, it could be shown for both steels that an increase of cyclic hardening potential, caused by longer aging times and different precipitation states of Cu, can be associated with increased fatigue strength. However, for an overall assessment of a material’s mechanical properties, both, hardness and cyclic hardening potential have to be considered.

High cycle fatigue behavior of high-Mn TWIP steel with different surface morphologies

Cyclic deformation behavior and fatigue life of high-Mn TWIP steel with different surface morphologies was investigated. Specimens with different surface morphologies, namely polished, down and up milled, were loaded in the HCF regime. The surface morphologies were analyzed by cyclic indentation tests, XRD, SEM and FIB methods. Milling led to differences in topographies, residual stresses and microstructures. Fatigue tests at various stress amplitudes were conducted in the HCF regime, investigating how these surface morphologies affect the cyclic deformation behavior and consequently fatigue life. The results show a significant influence of the surface morphologies on the material’s behavior under cyclic loading.

Online Acoustic Emission Monitoring of Cyclic Ball Indentation Testing - Correlation with Hysteresis Area Response

Fatigue has been one of the most researched subjects as most of the critical component failures are traced to fatigue. While fatigue data generation for design purposes is carried out using ASTM or equivalent standard specimens, the use of miniature or small specimens to estimate the fatigue properties is considered as a tool for extending the remaining life of in-service components. Cyclic automated ball indentation (Cyclic ABI) is one of the non-conventional test techniques used for fatigue performance assessment of pristine and in-service damaged materials. This method uses compression-compression cyclic loading of a flat specimen using a tungsten carbide spherical indenter; continuous monitoring of load-displacement (measured close to the indentation location) data provides an idea about the fatigue life of the material. Apart from this, hysteresis in load-displacement is used as an indicative energy parameter to detect fatigue failure in an Inconel 617 alloy using an off-line data analysis. To ensure on-line tracking of failure events, a specially tuned, miniature acoustic emission (AE) sensor was used during cyclic indentation testing. The AE parameters were extracted in the format of counts, absolute energy; the result processed in terms of cumulative counts, cumulative energy as well as first derivative of acoustic emission counts vs. fatigue cycles was used to cross-correlate failure events with other sensor responses. The failure cycles identified from AE were found to be in good agreement with the hysteresis area under the load-displacement curves, as well as extensometer displacement during cyclic loading.